Cladding defined transmission grating

ABSTRACT

Disclosed herein are techniques, methods, structures and apparatus for providing photonic structures and integrated circuits with optical gratings disposed within cladding layer(s) of those structures and circuits.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/664,614 filed Jun. 26, 2012 which isincorporated by reference in its entirety as if set forth at lengthherein.

TECHNICAL FIELD

This disclosure relates generally to the field of optical communicationsand in particular to techniques, methods and apparatus for opticallycoupling an optical fiber to a photonic integrated circuit.

BACKGROUND

Contemporary optical communications and other systems oftentimes requirethe optical coupling of an optical fiber to a photonic integratedcircuit (PIC). Such optical coupling is oftentimes difficult due—inpart—to the spot-size difference between the optical fiber mode(s) and awaveguide integrated within the PIC. Particularly difficult are thoseconfigurations involving a high index contrast platform such asSilicon-on-Insulator (SOI).

Accordingly, methods, structures or techniques that facilitate theoptical coupling of optical fiber to a PIC would represent a welcomeaddition to the art.

SUMMARY

An advance in the art is made according to an aspect of the presentdisclosure directed to techniques, methods and apparatus for opticallycoupling optical waveguides and optical structures through the effect ofcladding defined gratings. In an exemplary embodiment according to thepresent disclosure, such cladding defined structures include a coreregion, a cladding region adjacent to that core region, and an opticalgrating defined in that cladding region. In particular embodiments,multiple gratings of different types may be defined in different layersof the cladding region.

BRIEF DESCRIPTION OF THE DRAWING

A more complete understanding of the present disclosure may be realizedby reference to the accompanying drawings in which:

FIG. 1 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer sandwiched between a stack of claddinglayers;

FIG. 2 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer between a stack of cladding layersincluding a phase grating within a cladding layer according to an aspectof the present disclosure;

FIG. 3 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer sandwiched between a stack of claddinglayers including a transmission grating formed within a cladding layeraccording to an aspect of the present disclosure;

FIG. 4 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer between a stack of cladding layersincluding a transmission grating according to an aspect of the presentdisclosure;

FIG. 5 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer between a stack of cladding layersincluding a transmission grating according to an aspect of the presentdisclosure;

FIG. 6 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer between a stack of cladding layersincluding multiple transmission gratings according to an aspect of thepresent disclosure;

FIG. 7 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer between a stack of cladding layersincluding multiple transmission gratings according to an aspect of thepresent disclosure;

FIG. 8 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer between a stack of cladding layersincluding multiple transmission gratings according to an aspect of thepresent disclosure;

FIG. 9 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer between a stack of cladding layersincluding multiple transmission gratings that are shifted relative toone another according to an aspect of the present disclosure;

FIG. 10 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer between a stack of cladding layerswherein the core layer is locally thinned to control confinement of theoptical mode according to an aspect of the present disclosure;

FIG. 11 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer between a stack of cladding layerswherein core layer is locally thinned to control the confinement of theoptical mode and cladding layers 3 and 5 contain a transmission gratingwhile layers above layer 4 are planar according to an aspect of thepresent disclosure;

FIG. 12 shows a field intensity plot of grating in the cladding wherelight is coming from the top and bottom with a pi phase differenceaccording to an aspect of the present disclosure;

FIG. 13 shows a field intensity plot of grating in the cladding wherelight is coming from the top and bottom with no phase differenceaccording to an aspect of the present disclosure;

FIG. 14 shows a two dimensional transmission grating defined in thecladding region for vertical coupling to an optical fiber where light iscoupled into four waveguides according to an aspect of the presentdisclosure;

FIG. 15 shows a schematic cross-section of a grating coupler wherein thegrating is defined in the cladding layer according to an aspect of thepresent disclosure; and

FIG. 16 shows the coupling efficiency of a grating coupler from theoptical fiber to one waveguide (left), from two opposite waveguides tothe optical fiber (right), according to an aspect of the presentdisclosure.

DETAILED DESCRIPTION

The following merely illustrates the principles of the disclosure. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the disclosure and are includedwithin its spirit and scope.

Furthermore, all examples and conditional language recited herein areprincipally intended expressly to be only for pedagogical purposes toaid the reader in understanding the principles of the disclosure and theconcepts contributed by the inventor(s) to furthering the art, and areto be construed as being without limitation to such specifically recitedexamples and conditions.

Moreover, all statements herein reciting principles, aspects, andembodiments of the disclosure, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently-known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat the diagrams herein represent conceptual views of illustrativestructures embodying the principles of the invention.

In addition, it will be appreciated by those skilled in art that anyflow charts, flow diagrams, state transition diagrams, pseudocode, andthe like represent various processes which may be substantiallyrepresented in computer readable medium and so executed by a computer orprocessor, whether or not such computer or processor is explicitlyshown.

In the claims hereof any element expressed as a means for performing aspecified function is intended to encompass any way of performing thatfunction including, for example, a) a combination of circuit elementswhich performs that function or b) software in any form, including,therefore, firmware, microcode or the like, combined with appropriatecircuitry for executing that software to perform the function. Theinvention as defined by such claims resides in the fact that thefunctionalities provided by the various recited means are combined andbrought together in the manner which the claims call for. Applicant thusregards any means which can provide those functionalities as equivalentas those shown herein. Finally, and unless otherwise explicitlyspecified herein, the drawings are not drawn to scale.

Thus, for example, it will be appreciated by those skilled in the artthat the diagrams herein represent conceptual views of illustrativestructures embodying the principles of the disclosure.

A number of definitions are useful to facilitate one's understanding ofthe disclosure.

Effective refractive index of the optical mode: the phase change perunit length of the optical mode divided by the wave number in vacuum.

Effective wavelength: the wavelength in vacuum divided by the effectiverefractive index of the optical mode.

Core region: the stack of layers where the outer layers have arefractive index higher than the effective refractive index of theoptical mode and where a substantial part of the energy of the mode islocated.

Cladding region: the stack of layers surrounding the core region andhaving a refractive index lower than the effective refractive index ofthe optical mode

A Layer: a layer which has the same refractive index locally in thevertical dimension.

Effective refractive index of a layer: the weighted refractive index ofa layer over an area of at least one effective wavelength squared.

First order diffraction: the first order diffraction of grating as knownby somebody skilled in the art of grating designs.

Transmission grating: also called a binary phase grating: a periodicstructure where the transmitted light experiences a different opticalpath length thus introducing a phase shift between the light exiting thegrating at different positions.

With these definitions in place, and by way of some additionalbackground, we note as before that coupling light from an optical fiberinto a chip containing an integrated photonic circuit is a non-trivialexercise due—in part—to spot-size difference(s) between the opticalfiber mode and an integrated waveguide can be very large. Especially inthose configurations involving high index contrast platforms such asSilicon-on-Insulator (SOI), highly efficient coupling is a dauntingtask.

One known solution is to employ a horizontal spot-size convertor at theedge of the chip (See, e.g., T. Shoji, T. Tsuchizawa, T. Watanabe, K.Yamada, and H. Morita, “Low loss mode size converter from 0.3 μm squareSi waveguides to singlemode fibres,” Electronics Letters, vol. 38, no.25, p. 1669, 2002). This approach—while very efficient—is unfortunatelyquite elaborate since it requires wafer dicing and sometimes facetpolishing. Additionally with this approach, the number of couplingsections is limited by the circumference of the chip.

An alternative coupling solution shown in the art is one described asout-of-plane coupling. (See, e.g., D. Taillaert et al., “An out-of-planegrating coupler for efficient butt-coupling between compact planarwaveguides and single-mode fibers,” IEEE Journal of Quantum Electronics,vol. 38, no. 7, pp. 949-955, July 2002. Out-of-plane coupling enablesoptical wafer probing in a manner similar to electrical wafer probing.

With out-of-plane coupling techniques, an out-of-plane coupler is aperiodically diffractive structure formed into a waveguide core anddiffracts light out of the chip into the substrate and superstrate.Since out-of-plane coupling is a type of diffraction based coupling, itis inherently narrowband and polarization dependent.

Consequently common problem for out-of-plane grating couplers is thatthe light is diffracted to multiple diffraction orders of the gratingthereby limiting the coupling efficiency. Particularly for thosesituations exhibiting perfect vertical coupling, the second orderreflection introduces a large loss factor.

However, by exciting the grating from both sides, the second orderreflection can be compensated by the transmission of the light in theother direction (See, e.g., C. R. Doerr et al., “Monolithic Polarizationand Phase Diversity Coherent Receiver in Silicon,” Journal of LightwaveTechnology, vol. 28, no. 4, pp. 520-525, February 2010. Such“bi-directional” couplers have an added advantage of having a bettermode overlap with respect to a uniform grating coupler excited from onlyone side.

Those skilled in the art will appreciate that a polarization splittinggrating coupler is oftentimes used to couple to a near-vertical fiberfor both polarizations (See, e.g., A Mekis, A. Dodabalapur, R. E.Slusher, and J. D. Joannopoulos, “Two-dimensional photonic crystalcouplers for unidirectional light output.,” Optics letters, vol. 25, no.13, pp. 942-4, July 2000.; D. Taillaert, P. I. Borel, L. H. Frandsen, R.M. De La Rue, and R. Baets, “A compact two-dimensional grating couplerused as a polarization splitter,” IEEE Photonics Technology Letters,vol. 15, no. 9, pp. 1249-1251, September 2003). With such polarizationsplitting couplers however, the electrical field of one of theorthogonal fiber polarizations is not in-plane therefore polarizationdependent loss (PDL) is introduced.

As may be appreciated, a bi-directional, perfectly vertical coupler isbetter suited for a 2-dimensional polarization splitting gratingcoupler. More specifically, when using a perfectly vertical fiber and abi-directional polarization splitting grating coupler, the symmetry ofthe fiber is preserved and no PDL is introduced by the coupling section.

With respect to a 2D polarization splitting grating coupler in SOI witha Si core layer of 220 nm and a central wavelength of 1550 nm, suchstructure typically has a 1 dB bandwidth on the order of 25 nm and acoupling efficiency of around 25%.

According to an aspect of the present disclosure, a grating coupler isdisclosed that advantageously improves the 1 dB bandwidth to 45 nmthereby easily covering the whole C-band while increasing the couplingefficiency to 80% for both polarizations. Of further advantage, agrating coupler according to the present disclosure considerablyincreases the tolerance to process variations while simplifying thefabrication process flow.

According to an aspect of the present disclosure, the position of agrating coupler is changed from the core layer to the cladding layer. Asa result, instead of etching a grating into a core waveguide layer, thegrating is etched it the cladding layer. As a result the part of theoptical waveguide mode that interacts with the grating so constructed isthe evanescent field instead of the core field.

As will become readily apparent, our approach to grating couplerconstruction according to the present disclosure has numerousadvantageous. More specifically when a grating is etched into awaveguide core or is in very close proximity to the waveguide core, anychange in the grating such as the etch depth, will influence both thegrating strength and effective refractive index of the optical mode. Insharp contrast and according to the present disclosure, by positioning agrating far enough from the waveguide core layer, the grating itselfwill impart minimal influence on the waveguide mode and thus on theeffective refractive index of the mode.

Importantly, and according to an aspect of the present disclosure, thestrength of a grating is not determined by the etch depth but by theoptical confinement of the waveguide mode. By controlling the core layerthickness, the amount of light in the evanescent field can be controlledand thereby limit the grating strength.

According to the present disclosure, vertical dimensions of the gratinghave minimal influence on the spectral response of the grating coupler(besides the grating directionality) and therefore one can optimize thethickness in such a way that the grating also acts as a transmissiongrating which only favors the first order diffraction in thesuperstrate. This characteristic advantageously enables couplingefficiencies of 80% in the case of a perfectly vertical bi-directionalpolarization splitting grating coupler. Furthermore, the optimal Si corelayer thickness is 150 nm which is considerably lower than the typical220 nm. This reduces the effective refractive index of the waveguidemode under the grating and increases the 1 dB-bandwidth of the gratingcoupler up to 45 nm. (See., e.g., C. R. Doerr, L. Chen, Y. K. Chen, andL. L. Buhl, “Wide bandwidth silicon nitride grating coupler,” PhotonicsTechnology Letters, IEEE, vol. 22, no. 19, pp. 1461-1463, 2010)

As may be further appreciated, integrated photonic waveguides requiregratings for all kinds of photonic functionalities. In a typicalsituation, one is interested in the diffraction of the light accordingto a certain order of the grating. For example a Bragg reflector gratingdiffracts the light according the first order of the grating obtaining aphase match with the backwards travelling optical mode in the waveguide.Other exemplary functionalities include grating couplers wherein thediffracted light is phase matched to a plane wave in the top or bottomcladding, achieving out-of-plane coupling. This can be used to coupleto, for example, an optical fiber, laser or photodetector.

Typically one defines the grating in the optical core region of thewaveguide as explained in the Definitions section. According to thepresent disclosure however, we deliberately define the grating in acladding layer and use it as a transmission grating in order to favor acertain diffraction order and achieve highly efficient coupling. Oneparticular embodiment according to the present disclosure is one whereinthe transmission grating is only defined in the top cladding or in thebottom cladding in order to favor the upwards or the downwardsdiffraction order respectively.

With reference now to FIG. 1, there it shows a schematic cross-sectionalview of a photonic waveguide including a waveguide core layer (1)sandwiched or otherwise positioned or formed between a stack of claddinglayers (2-9) as one may find in the current art.

FIG. 2 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer (1) positioned between a stack ofcladding layers (2-9) including a phase grating (10) according to anaspect of the present disclosure. As may be observed from this FIG. 2,the grating (10) is formed within cladding layer 2.

Similarly, FIG. 3 shows a schematic cross-sectional view of a photonicwaveguide including a waveguide core layer (1) positioned between astack of cladding layers (2-9) including a transmission grating (10)according to yet another aspect of the present disclosure. As may beobserved from this FIG. 3, the grating (10) is formed within claddinglayer 3.

FIG. 4 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer (1) positioned between a stack ofcladding layers (2-9) including a transmission grating (10) according toan aspect of the present disclosure. As may be observed from this FIG.4, the grating (10) is formed within cladding layer 5.

Up to this point in the discussion, the grating(s) have been depicted“above” the waveguide core in the figures. And while we have notassigned any direction or overlying/underlying relationships to thelayers depicted in the figures, for completeness we show now in FIG. 5 aschematic cross-sectional view of a photonic waveguide including awaveguide core layer (1) positioned between a stack of cladding layers(2-) including a transmission grating (10) according to an aspect of thepresent disclosure. As may be observed from this FIG. 5, the grating(10) is formed within cladding layer 8.

FIG. 6 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer (1) positioned between a stack ofcladding layers (2-9) including multiple transmission gratings (10, 11)according to an aspect of the present disclosure. As may be observedfrom this FIG. 6, the gratings (10, 11) are formed within claddinglayers 2 and 4 respectively.

Similarly, FIG. 7 shows a schematic cross-sectional view of a photonicwaveguide including a waveguide core layer (1) positioned between astack of cladding layers (2-9) including multiple transmission gratings(10, 11) according to an aspect of the present disclosure. As may beobserved from this FIG. 6, the gratings (10, 11) are formed withincladding layers 3 and 5 respectively.

FIG. 8 shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer (1) positioned between a stack ofcladding layers (2-9) including multiple transmission gratings (10-11)according to an aspect of the present disclosure. As may be observedfrom this FIG. 8, the gratings (10, 11) are formed within adjacentcladding layers 3 and 4 respectively.

A variation of the configuration depicted in FIG. 8 is shown in FIG. 9which shows a schematic cross-sectional view of a photonic waveguideincluding a waveguide core layer (1) positioned between a stack ofcladding layers (2-9) including multiple transmission gratings (10, 11)according to an aspect of the present disclosure. As may be observedfrom this FIG. 9, the gratings (10, 11) are formed within adjacentcladding layers 3 and 4 respectively, and are shifted relative to oneanother.

One particular embodiment of this invention is a photonic circuit with atransmission grating in a cladding layer and where the optical corestack is locally thinned to control the effective grating strength.

More particularly, FIG. 10 shows a schematic cross-sectional view of aphotonic waveguide including a waveguide core layer (1) positionedbetween a stack of cladding layers (2-9) wherein the core layer (1) islocally thinned to control confinement of the optical mode according toan aspect of the present disclosure. As may be observed from this FIG.10, the gratings (10, 11) are formed within cladding layers 3 and 5respectively.

Another particular embodiment according to the present disclosureemploys a “cladding grating” such as that disclosed, to prepare agrating coupler where light is coupled—for example—from an opticalwaveguide to an optical fiber. The grating coupler can be made in atwo-dimensional manner such that it may be excited from four sides. Asmay be appreciated, by exciting the grating from opposite sides and bychoosing a certain phase difference, the grating strength and gratingreflection can be controlled.

Yet another exemplary embodiment according to the present disclosure isone wherein the cladding grating is positioned on top of a waveguidewhere light is coupled from a forward propagating mode into a backwardpropagating mode. One particular benefit of structures constructedaccording to the present disclosure is that the thickness of thetransmission grating may advantageously be changed without affecting thegrating strength in a substantial way. Therefore the grating strengthand the particular phase shift required can be optimized independentlyof one another.

Furthermore, more than one grating can be introduced in the claddinglayers, wherein the individual gratings have separate functionalities.More particularly, a first grating positioned close to the core stackcan serve as a diffraction grating while a second grating further fromthe core stack serves as a transmission grating. In this manner a singlegrating can be looked at as two gratings where the structure positionedclosest to the waveguide acts as a scattering grating while the top partintroduces a phase shift between light scattered at the differentgrating areas.

Another exemplary embodiment of a structure according to the presentdisclosure is depicted in FIG. 11. As may be observed, FIG. 11 shows aschematic cross-sectional view of a photonic waveguide including awaveguide core layer (1) positioned between a stack of cladding layers(2-9) wherein core layer (1) is locally thinned to control theconfinement of the optical mode according to an aspect of the presentdisclosure. As may be further observed from this FIG. 11, the gratings(10, 11) are formed within cladding layers 3 and 5 respectively whilelayers above layer 4 are planar.

With continued reference to FIG. 11, the exemplary structures showntherein may comprise—a Si substrate layer (layer 8); a buried oxidelayer on the order of a micron in thickness (layer 7); a Si core layerof substantially 220 nm thickness (layer 1); a dielectric layer of oxidewith a thickness of substantially 40 nm (layer 2); an oxide layer (layer4); poly-Si structures with a height of around 220 nm in the claddinglayers (3 and 5) which form a grating structure and where the meanrefractive index is locally smaller than the effective refractive indexof the mode.

FIG. 12 shows a field intensity plot of grating in the cladding wherelight is coming from the top and bottom with a pi phase differenceaccording to an aspect of the present disclosure, while FIG. 13 shows afield intensity plot of grating in the cladding where light is comingfrom the top and bottom with no phase difference according to an aspectof the present disclosure.

Examples of 2D structures constructed according to the presentdisclosure are depicted in FIGS. 14 and 15.

FIG. 14 shows a two dimensional transmission grating defined in thecladding region for vertical coupling to an optical fiber where light iscoupled into four waveguides according to an aspect of the presentdisclosure.

FIG. 15 shows a schematic cross-section of a grating coupler wherein thegrating is defined in the cladding layer according to yet another aspectof the present disclosure.

One particular advantage to structures constructed according to thepresent disclosure is that such structures exhibit a coupling efficiencyof 80% to an optical fiber. Of further advantage, such efficiencies maybe realized for every polarization state of light so coupled.

FIG. 16 shows two plots exhibiting the coupling efficiency of a gratingcoupler from the optical fiber to one waveguide (left), from twoopposite waveguides to the optical fiber (right), according to an aspectof the present disclosure. As may now be appreciated, such couplingefficiencies are in fact a welcome addition to the art.

At this point, those skilled in the art will appreciate that a number ofadvantages emerge with respect to structures made according to thepresent disclosure. In particular, by defining a grating in the claddinglayer with a refractive index lower than the effective refractive indexof the optical mode, one assures that the mode stays substantiallyconfined in the core layers.

Additionally, by controlling the distance of the layer containing thegrating with respect to the stack of core layers, one can control thegrating strength or the amount of light per unit length that getsdiffracted by the grating.

By controlling the thickness of the core layers locally, one can tunethe optical confinement of the mode and thus the grating strength or theamount of light that gets diffracted at the grating in a cladding layer.

Furthermore, by introducing a transmission grating above or under thestack of core layers, the vertical symmetry is broken and by choosingthe optimal phase shift, substantially efficient coupling to exactly onefirst order diffraction can be achieved.

Still further, by thinning down the stack of core layers locally, onecan achieve a set of gratings with different grating strength withoutchanging the grating itself. Thereby gratings with different gratingstrengths at different places on the chip can be achieved.

Additionally, by introducing more than one grating in the claddinglayers, the diffraction function of the grating and the phase shiftfunction of the grating can be functionally separated.

By having the grating in the cladding layer, the effective refractiveindex of the cladding layer containing the grating will have a minimalinfluence on the effective refractive index of the mode, making thewhole structure more robust.

By exciting the grating from opposite sides a standing wave pattern canbe formed thereby having influence on the grating strength. By changingthe phase difference between the light coming from opposite sides, onecan successfully control the grating strength. (see FIGS. 12 and 13).

Finally, by exciting the grating from opposite sides the reflection ofthe grating can be compensated and minimized, and by choosing anappropriate cladding layer or cladding layers between the cladding layercontaining the grating and the core stack, the intermediate claddinglayer(s) can be used as an etchstop layer for defining the grating inthe cladding layer using a selective etching method. This is particularadvantage with respect to timed etch gratings in terms of robustness.

Those skilled in the art will readily appreciate that while the methods,techniques and structures according to the present disclosure have beendescribed with respect to particular implementations and/or embodiments,those skilled in the art will recognize that the disclosure is not solimited. Accordingly, the scope of the disclosure should only be limitedby the claims appended hereto.

The invention claimed is:
 1. A photonic structure, comprising: a core region; and a first optical grating adjacent to the core region, the first optical grating defined by a plurality of disconnected pillars of a first cladding material separated by at least one second material; and a second optical grating formed in a second cladding material, the first and second optical gratings being disposed on a common side of the core region such that the first optical grating is disposed between the second optical grating and the core region, wherein the first and second cladding materials have an effective refractive index lower than an effective refractive index of at least one layer in the core region.
 2. The photonic structure according to claim 1, wherein said second optical grating is a transmission grating, said transmission grating exhibiting an effective refractive index lower than the at least one layer in the core region.
 3. The photonic structure of claim 2, wherein the transmission grating overlaps partly with an evanescent field of an optical mode in the core region.
 4. The photonic structure of claim 3, wherein the core region has a non-uniform thickness.
 5. The photonic structure of claim 4, wherein the transmission grating diffracts light to the first order diffraction.
 6. The photonic structure of claim 5 wherein the first order diffraction is phase matched to an optical waveguide mode.
 7. The photonic structure of claim 5, wherein the first order diffraction is a plane wave in a cladding layer of the photonic structure.
 8. The photonic structure of claim 1, wherein the first and second optical gratings are laterally offset from one another.
 9. The photonic structure of claim 8, wherein the first and second optical gratings are formed in adjacent layers.
 10. The photonic structure of claim 1, wherein the first and second optical gratings are formed in non-adjacent layers.
 11. The photonic structure of claim 1, wherein the first optical grating is a diffraction grating.
 12. A photonic structure, comprising: a core region; and an optical grating adjacent to the core region, the optical grating defined by a plurality of disconnected pillars of a first cladding material separated by at least one second material, the optical grating having a lateral extent from a first pillar of the plurality of disconnected pillars to a last pillar of the plurality of disconnected pillars, wherein the core region includes a first portion of a first, substantially uniform thickness substantially aligned laterally with the optical grating and a second portion of a second thickness greater than the first thickness laterally outside the lateral extent of the optical grating.
 13. The photonic structure of claim 1, wherein the first optical grating is a two dimensional grating.
 14. The photonic structure of claim 1, wherein the at least one second material is silicon oxide.
 15. The photonic structure of claim 1, wherein a first pillar of the plurality of disconnected pillars is multi-sided, and wherein all but one side of the first pillar directly contacts the at least one second material.
 16. The photonic structure of claim 1, wherein the plurality of disconnected pillars comprises at least ten pillars.
 17. The photonic structure of claim 12, wherein the plurality of disconnected pillars are arranged in a planar configuration within a common layer of the second material.
 18. The photonic structure of claim 12, wherein the second portion of the core region has a substantially uniform thickness. 